Chemical-mechanical planarization of barriers or liners for copper metallurgy

ABSTRACT

A tantalum-based liner for copper metallurgy is selectively removed by chemical-mechanical planarization (CMP) in an acidic slurry of an oxidizer such as hydrogen peroxide, deionized water, a corrosion inhibitor such as BTA, and a surfactant such as Duponol SP, resulting in a high removal rate of the liner without appreciable removal of the exposed copper and with minimal dishing.

FIELD OF THE INVENTION

[0001] The present invention relates generally to integrated circuitmetallization, and more particularly to a methods of formingmetallization by planarizing copper wiring wherein a liner is used undera copper layer.

BACKGROUND OF THE INVENTION

[0002] In the field of integrated circuit manufacturing, it is wellknown that significant density advantages result from forming planarmetallization patterns which interconnect one or more transistors,capacitors, resistors, and other semiconductor electronic componentsformed on a wafer. One of the significant trends in the industry isproduce such planar metallization patterns using so-called“Chemical-Mechanical Planarization” or “Chem-Mech Polish” or just “CMP”techniques. In CMP, the front side of a semiconductor wafer is heldagainst a rotating polish wheel, and a polishing slurry is introducedthat facilitates the planarization and partial removal of a metal layeron the wafer through a combination of chemical reaction and physicalabrasion. See for example U.S. Pat. No. 4,944,836, issued Jul. 31, 1990to Beyer et al. and assigned to the assignee of the present invention,which teaches the CMP of metals or insulators to form coplanarinsulator/metal films.

[0003] In addition to the general advantages afforded by CMP, particulardensity advantages result from forming an integrated metallurgy inpre-planarized passivation. That is, as shown in U.S. Pat. 4,789,648,issued Dec. 6, 1988 to Chow et al and assigned to the assignee of thepresent invention (hereinafter the “Chow patent”), which teaches themethod of forming coplanar imbedded lines in insulators where the linesinclude integral vias to underlying conductive layers.

[0004] Normally, the horizontal (line) and vertical (via) portions ofthe metallurgy are formed using two separately formed layers of metal;the Chow patent maximizes conductivity by eliminating the interlayerinterface that normally exists between the horizontal and verticalportions of the metallurgy structure.

[0005] In the prior art, it is well known to utilize aluminum alloys ortungsten as the metallurgy for integrated circuits. However, theconductivity characteristics of these materials may not be sufficient asthe density of semiconductor chips increases below 0.4 microns. Thesemetallurgies are typically deposited on the wafer utilizing chemicalvapor deposition, sputtering or other directional deposition techniques.As chip area is reduced the aspect ratio (that is, the ratio of heightrelative to width) of vias, or openings, formed through passivationincreases making it difficult to deposit metal into high aspect rationvias. The result can be the formation of voids in metals depositedresulting in increased resistance or even failure to form usefulcontacts. In addition, at smaller geometries metal lines become moresusceptible to electromigration-induced faults. Thus, the semiconductorindustry has recently emphasized the development in the coppermetallurgies to replace aluminum-based metallurgy. Although copper haslow resistivity and higher electromigration resistance than aluminum, ithas no standard deposition technique. No fewer than six different typesof depositions methods have been investigated, including chemical vapordeposition, sputtering, evaporation, plasma CVD, electroless andelectroplating. Each has its advantages and disadvantages. In addition,several problems are required to be solved before a manufacturablecopper technology can be defined.

[0006] Copper, which is highly sensitive to corrosion does not form aself-passivating oxide as does aluminum, presents unique challenges,particularly in the area of CMP.

[0007] Initial methods of implementing copper in semiconductor devicesand planarization by CMP emphasized only the etching and abrasion ofcopper as evidenced by slurries such as Various slurries for CMP ofcopper have been proposed in the prior art. These techniquesinclude thefollowing: water, a solid abrasive, an oxidant from one of HNO3, H2SO4,and AgNO3 as taught in U.S. Pat. No. 5,354,490 to, Yu et al.; and water,a solid abrasive, an oxidant: HNO3 or NH4OH and KMnO4 with a H2O2buffering agent as taught in the article “Initial study on copper CMPslurry chemistries,” Ronald Carpio et al., thin solid films, Vol. 266,No. 2, 1 October 1995, pp. 238-244.

[0008] Various reagents have been proposed as additives to copper CMPslurries to (1) retard the polishing/etch rate of interlevel dielectricssuch as butanol as taught be U.S. Pat. No. 5,614,444 to Farkas et al. or(2) to retard the polish/etch rate of copper such as addingbenzotriazole (BTA) as taught in U.S. Pat. No. 5,770,095 to Sasaki etal. or the article “Alkaline Formulations for Chemical MechanicalPolishing of Copper Utilizing Azole Passivation,” anonymous, IBMTechnical Disclosure Bulletin, Vol. 37, No. 10, October 1994, p. 187.

[0009] Because copper has a high diffusion rate in some dielectrics,particularly silicon dioxide, some form of barrier layer betweeninterlevel dielectrics and copper metallurgy is required. Variousbarrier materials have been proposed including refractory metals such astitanium (Ti), tantalum (Ta), tungsten (W), compounds such as titaniumnitride and tantalum nitride, alloys such as TiW, doped metals such astitanium or tantalum doped with nitrogen (Ti(N) or Ta(N)) and bilayerssuch as Ti/W or Ta/TaN have been proposed by any of several references.

[0010] CMP methods proposed to be used to polish or planarize coppermetallurgy including under lying barrier materials have in manyinstances comprised a single slurry and polishing step as taught in U.S.Pat. No. 5,447,887 to Filipiak et al. (single step tantalum, titanium ortitanium-tungsten barrier layer, slurry unidentified); U.S. Pat. No.5,575,885 to Hirabayashi et al. (alkaline slurry with titanium, titaniumnitride, niobium, tungsten or a copper-tantalum alloy); U.S. Pat. No.6,612,254 to Mu et al. (ammonium hydroxide, silica and water with abarrier layer of Titanium nitride) and the article “CHEMICAL-MECHANICALPOLISHING OF COPPER IN ACIDIC MEDIA” by Q. Luo et al., 1996 CMP-MICConference, Feb. 22-23, 1996, pp. 145-151 (Acidic ferric nitrate, BTA,poly ethlyene glycol surfactant, alumina and water with atitanium-tungsten alloy barrier layer).

[0011] Recently the evolution of copper CMP has included two-stepprocesses as taught by U.S. Pat. 5,676,587 to Landers et al. (copperpolish using alumina-based slurry selective to the copper and asilica-based slurry selective to the barrier layer of Ta/TaN) or athree-step process as taught by U.S. Pat. No. 5,516,346 to Cadien et al.(separate slurries for each of copper, tungsten and titanium nitride).

[0012] Most recently a barrier layer comprising a first layer ofhexagonal phase tantalum nitride and a second layer of alpha-phasetantalum has been proposed in European Patent Application EP 0751566 A2to Cabral et al. published Jan. 2, 1997.

[0013] Finally, co-pending U.S. Provisional application No. 60/105,470filed Oct. 23, 1998 teaches a slurry for copper comprising water,alumina, an oxidizing agent (ferric nitrate), a copper passivating agent(BTA) and a surfactant (Duponol SP) which has proved to be highlyselective to copper over the liner of Cabral et al. European applicationEP 0 751 566 A2, as well as other tantalum-based liners.

SUMMARY OF THE INVENTION

[0014] In general, the prior art set forth above deals with polishing ofcopper with and without barrier materials in conventional metal line orstud via applications. However, when polishing metal in a dual Damasceneenvironment where both lines and studs may be formed, as shown in theChow patent, particular challenges are presented that must be addressedin the CMP process. As shown in FIG. 1, oxide 10 has narrow openings 12that define vias, and wide openings 14 that define the metal lines. Eachopening is filled with liner 20 and copper 22. As initially deposited,copper layer 22 extends over portions of barrier layer 20 and is removedby a separate CMP step as described in co-pending ProvisionalApplication Serial No. 60/105,470 filed Oct. 23, 1998, the teachings ofwhich are incorporated herein by reference. The removal of the overlyingcopper results in the structure as shown in FIG. 2.

[0015] In the invention, the portions of the metal layers 20 and 22formed above the upper surface of oxide 10 are removed by CMP, utilizingthe polishing slurry of the invention. During this operation, two goalsare to be achieved. One is to maximize the removal rate of the copper,because the faster the polish process can be achieved the more waferscan be processed per unit time, decreasing the overall cost. Anothergoal is to remove the copper without appreciably removing the liner,because removal of the liner could lead to partial removal of theunderlaying oxide, introducing non-planarity in the final surface andallows the switching to another slurry more selective to the liner.These goals of the copper CMP process (high removal rate of copper,without appreciable removal of the underlaying liner) can often beincompatible; in the prior art, removal rate of the bulk metal is oftensacrificed in order to prevent excessive liner removal.

[0016] In addition, the copper slurry should minimize dishing anderrosion in the polished copper. Ideally, when the CMP process iscompleted, the upper surface of the copper would be coplanar with thesurrounding oxide. This result can be achieved for the copper in thenarrow openings 12. However, in the wide openings 14, the pressure ofthe polish pad on the wafer results in a bowing (or “dishing”) profile,in which the thickness of the copper at the center of the wide openingis less than the thickness at either end. In general, this structure isto be avoided, because it reintroduces non-planarity in the final BEOLstructure and reduces the thickness of lateral interconnects such asline structures 14. Errosion is the removal of the non-copper regionsbetween narrow lines in high density wiring areas. That is, it isundesirable for the planarization slurry to be yoo agressive in removingthe interlevel dielectric (ILD) between lines once the ILD has beenexposed.

[0017] Therefore, another object of the present invention is to providea CMP process that minimizes dishing and errosion. The specific goal isto prevent the difference between the thin and thick regions of thecopper within opening 14 from being greater than the thickness of theliner 14 disposed on the upper surface of the oxide 10.

[0018] In the invention, a liner or barrier layer for copper metallurgycomprising a refractory and/or its alloys and compounds is removed byCMP in an acidic slurry of an oxidizer such as hydrogen peroxide,deionized water, a corrosion inhibitor such as BTA, and a surfactantsuch as Duponol SP, resulting in a high removal rate of the linerincluding any remaining copper without appreciable removal of theunderlying insulator layer.

BRIEF DESCRIPTION OF THE DRAWING

[0019] The above and other aspects of the present invention will becomemore clear upon review of the detailed description of the invention asset forth below. In the description to follow, reference will be made tothe accompanying drawing.

[0020]FIG. 1 is a cross sectional view of a substrate with a layers ofcopper and liner thereon undergoing the CMP process of the invention.

[0021]FIG. 2 is a cross sectional view of a substrate following theremoval of the copper layer.

[0022]FIG. 3 is an enlarged sectional view of a portion of the surfaceof a semiconductor device following the removal of the liner layer.

DETAILED DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION

[0023] In the invention as shown in FIG. 1, the liner 20 is on the orderof 40-80 nm thick, and is comprised of tantalum. Specific materialsinclude tantalum alloysand tantalum-based compositions such as tantalumnitride, as well as other refractory metals and refractory metal alloyssuch as chromium and titanium/titanium nitride. A Ta-based barrier layerappears to provide the best combination of (a) promoting adhesion ofcopper to the oxide layer 10, and (b) preventing diffusion of copper,which is highly corrosive, into the underlaying silicon. In the practiceof the invention, the specific layer to be polished is a tantalum-basedlayer comprising a bilayer of tantalum nitride and tantalum, each about20-40 nm in thickness. Then the copper 22 is deposited onto the wafer,to a thickness (measured from the upper surface of oxide layer 10 to theupper surface of the copper layer) of approximately one micron. Thecopper layer can be formed using any deposition process that adequatelyfills the apertures oxide layer 10 without the formation of voids.

[0024] The upper portion of the copper layer is then planarized via CMPto remove those portions of the metal layers that overly the uppersurface of the passivation layer, so as to form a co-planarmetal/insulator surface. In practice, the workpiece of FIG. 1 is placedinto the wafer holder of a CMP tool. Any one of a number of known CMPtools, including those available from IPEC Corp. and Strasbaugh Corp.could be used to carry out the present CMP operation. The CMP tool has arotating polish wheel on which is mounted a polish pad. The CMP slurryof the invention is applied onto the polish pad, and the wafer holderbrings the wafer into contact with the rotating polish wheel. Inpractice, the wafer holder spins in a direction opposite to that of thepolish wheel. In the operation described below, typically the polishwheel rotates at a speed of approximately 75 revolutions per minute(RPM), and the wafer holder spins at a speed of approximately 50 RPM.Further, the wafer holder applies pressure to force the wafer againstthe polish pad. As will be described in more detail below, the inventorshave found that non-woven pads provide the best tradeoff between highremoval rates and low dishing and scratching. In the operation describedbelow, the applied pressure is approximately 5 pounds per square inch(PSI). In practice, these three variables (spin speed of the waferholder, rotation speed of the polish pad, pressure) are interrelated,and can and do vary as a function of e.g. the hardness and thickness ofthe layer to be polished.

[0025] In the slurry of the invention, solids such as silica particleshaving an average diameter at or below approximately 0.4 nm are used toprovide physical abrasion of the liner material in the reaction whileminimizing excessive scratching or other surface damage. The slurryincludes the following components:

[0026] 1) an oxidizer that promotes the removal of any residual copperremaining following the removal of the upper portion of the copperlayer, an oxidizer including oxidizers from the following list can beused with varied success: ferric nitrate, hydrogen peroxide, potassiumiodate, manganese oxide, ammonium hydroxide, ammonium persulphate,potassium persulphate, ammonium persulphate/sulfuric acid, potassiumpersulphate/sulfuric acid, ferric chloride/hydrochloric acid, chromicacid, chromic acid/hydrochloric acid, potassium bichromate/sulfuric acidand stearic acid;

[0027] 2) a buffering agent that inhibits copper oxidation of theunremoved copper forming the lines by setting up a competing reactionwith copper radicals, such as various benzotriazole compounds sold underthe trade name “BTA” (e.g. 1-H benzotriazole, 1-OH benzotriazole, 1-CH3benzotriazole, 5-CH3 benzotriazole, benzimidazole, 2 OH,2-methyl-benzimidazole, 5-Cl benzotriazole);

[0028] 3) a sulfated fatty acid surfactant that competes with BTA forcopper reaction sites to slow down Cu-BTA complexing, as will bedescribed in more detail below; and

[0029] 4) a surfactant, for example, a version of sodium lauryl sulfate,available commercially as one or more of the series of Duponols, such asDuponol SP.

[0030] Before describing the process in detail, the objectives of theinvention will be described with reference to FIG. 3, which shows anenlarged view of the top surface of a semiconductor substrate before andafter the removal of the liner from the surface. As in FIG. 2 the dashedlines show the surface before removal. In the practice of the invention,it is desired to remove all of the liner 20 overlying the insulator 10leaving the top surface of the deposited copper protruding slightlyabove the top surface of the insulator by removing portions of thesurrounding insulator oxide 10. That is, the final topography of thesubstrate is not planar but provides for the protrusion of the copperslightly above the background oxide layer. This is particularly true forfine lines, as described above.

[0031] In order to successfully remove the liner selectively to thecopper and underlying oxide, we found that we had to first consider theremoval of any residual copper which might remain on the linerfollowingthe initial copper removal step. This residual copper might be trappedin scratches formed before the deposition of copper and which is notremoved during the copper polish step. Thus, in addition to providingabrasive material for the Ta/TaN liner, one needs to provide an oxidizerfor any residual copper. The slurry for the liner needs also to includean inhibitor to prevent the etching of exposed copper forming thedesired lines and vias in the oxide-covered substrate. As shown in FIG.3, it is desirable to provide a finished surface wherein the copper isslightly raised above the planarized surface of the substrate.

[0032] In order to accomplish all of the above, it is necessary to beable to control the rate of polishing of the liner 22 and the insulator10 to be greater than the rate of polishing of copper. Accordingly, thefollowing basic formula was found to provide the optimum for removingthe Ta/TaN liner from a silicon dioxide passivated semiconductorsubstrate.

[0033] One liter of colloidal silica (Cabot SC-E) containing between 2and 30 percent solids by weight per liter of deionized water;

[0034] between 0 and 10 ml/l 30% aqueous hydrogen peroxide as anoxidizer;

[0035] between 1.5 to 6.0 ml/l Duponol surfactant; and

[0036] between 0,4 and 4.0 g/l BTA. Balanced to a pH of between 3.0 and7.5.

[0037] The preferred embodiment includes colloidal silica at 15% byweight, 3.0 ml/liter hydrogen peroxide, 3.0 ml/liter of Duponol SP and1.2 g/liter BTA, balanced to a pH of 4.5.

[0038] It should be understood by those skilled in the art that otherabrasives such as ceria and alumina could be used. The inventors foundthat the addition of Duponol SP was essential to enhance the polishingcharacteristics. While the prior art utilizes surfactants primarily topromote uniformity in the CMP slurry suspension, in the presentinvention Duponol SP is used more for its properties as a wetting agent(i.e. it decreases the surface tension of the slurry). Typically, whenusing surfactants to stabilize the concentration of particulate insuspension, higher molecular weight surfactants are used (for example,the above cited Clarkson paper teaches use of a surfactant having amolecular weight on the order of 1M). As will be described in moredetail below, in the present invention lower molecular weightsurfactants are used; the inventors stabilize particulate concentrationby controlling the pH of the slurry.

[0039] The inventors have found that controlling temperature during thepolish process is important to maximize the tantalum removal rate whileminimizing dishing of exposed copper. More specifically, the inventorshave found that the temperature of the slurry increases as the polishproceeds, due to the frictional forces inherent in the polish process aswell as the chemical components of the above-specified copper complexingreaction. The inventors have found that if the temperature rises aboveapproximately 100 degrees F., dishing results because the polishing padbegins to deform (i.e. the glass transition temperature of the pad isreached), and as a result the chemical component of the CMP processincreases. In practice, this temperature control is achieved byutilizing a water-cooled platen.

[0040] The inventors have found that the slurry of the invention can bepremixed (i.e. some slurries can only be mixed at the CMP tool due torapid degradation in stability), and the premixed slurry is stable for aweek or more. Most CMP slurries at the pH range of the invention tend tolose stability quickly.

EXAMPLE 1

[0041] A workpiece prepared as above was polished in a colloidal silicadispersion having particulate on the order of 0.4 microns or less,available from Cabot Inc. as SC-E. This grain size appears to providethe optimum tradeoff between enhanced removal rates and low removal ratein the barrier film. The slurry included 18 liters deionized water (15%silica), 1.2 grams/liter BTA, 3 grams/liter 30% hydrogen peroxide, and1.2 ml/liter Duponol SP, such that the total slurry had a pH ofapproximately 4.5. The CMP polish pad used was a non-woven pad,consisting of synthetic fibers in a polymer binder. The table speed was75 RPM, and the spin speed of the wafer holder was 50 RPM. The wafer washeld against the polish pad at a pressure of 6 PSI. The resultingremoval rate was approximately 40 nm/minute of tantalum, 25 nm perminute of silicon dioxide and 5 nm/minute copper.

[0042] In addition to polishing tantalum-based liner materials, theslurry of the present invention is believed to be applicable to othertantalum alloys and other refractory liners that would be polished inaccordance with the reaction kinetics described above.

[0043] It should be understood that the above and other modifications inthe practice of the invention can be made to the preferred embodiment asset forth above without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A slurry for polishing a barrier layer forcopper-based metallurgy, comprising an oxidizing agent for oxidizingcooper, a cooper oxidation inhibitor, and an additive that appreciablyregulates complexing between copper and the oxidation inhibitor.
 2. Theslurry as recited in claim 1, wherein said oxidizing agent is selectedfrom the group consisting of ferric nitrate and compounds thereof,hydrogen peroxide, potassium iodate, manganese oxide, ammoniumhydroxide, ammonium persulphate, potassium persulphate, ammoniumpersulphate/sulfuric acid, potassium persulphate/sulfuric acid, ferricchloride/hydrochloric acid, chromic acid, chromic acid/hydrochloricacid, potassium bichromate/sulfuric acid, and stearic acid.
 3. Theslurry as recited in claim 1, wherein said oxidizing agent compriseshydrogen peroxide.
 4. The slurry as recited in claim 1, wherein saidoxidation inhibitor is selected from the group consisting of 1-Hbenzotriazole, 1-OH benzotriazole, 1-CH3 benzotriazole, 5-CH3benzotriazole, benzimidazole, 2 OH, 2-methyl-benzimidazole, and 5-Clbenzotriazole.
 5. The slurry as recited in claim 1, wherein saidoxidation inhibitor comprises a benzotriazole.
 6. The slurry as recitedin claim 1, wherein said additive is comprised of a sulfated fatty acid.7. The slurry as recited in claim 6, wherein said sulfated fatty acidhas a molecular weight less than approximately
 300. 8. The slurry asrecited in claim 7, wherein said sulfated fatty acid is selected fromthe group consisting of sodium octyl sulfate, Duponol SP, and DuponolWN.
 9. The slurry as recited in claim 1, wherein said additive comprisesDuponol SP.
 10. The slurry as recited in claim 1, further comprisingcolloidal silica.
 11. The slurry as recited in claim 10, wherein saidcolloidal silica has particulate having a size less than approximately0.4 microns.
 12. The slurry as recited in claim 1, wherein said slurryhas a pH of approximately 2.0 to 7.5.
 13. The slurry as recited in claim12, wherein said slurry has a pH of approximately 4.5.
 14. A CMP slurryfor polishing a diffusion barrier layer liner for a layer of copper or acopper alloy in a semiconductor substrate, said slurry providing a firstremoval rate of said liner and a second removal rate of copper, saidfirst removal rate being about eight times greater than said secondremoval rate, comprising a copper oxidizing agent, a copper oxidationinhibitor, and an additive that appreciably regulates complexing betweencopper and the oxidation inhibitor.
 15. The slurry as recited in claim14, wherein said oxidizing agent is selected from the group consistingof hydrogen peroxide, potassium iodate, manganese oxide, ferric nitrate,ammonium hydroxide, ammonium persulphate, potassium persulphate,ammonium persulphate/sulfuric acid, potassium persulphate/sulfuric acid,ferric chloride/hydrochloric acid, chromic acid, chromicacid/hydrochloric acid, potassium bichromate/sulfuric acid, and stearicacid.
 16. The slurry as recited in claim 15, where in said oxidizingcomprises hydrogen peroxide.
 17. The slurry as recited in claim 14,wherein said oxidation inhibitor is selected from the group consistingof 1-H benzotriazole, 1-OH benzotriazole, 1-CH3 benzotriazole, 5-CH3benzotriazole, benzimidazole, 2 OH, 2-methyl-benzimidazole, and 5-Clbenzotriazole.
 18. The slurry as recited in claim 17, wherein saidoxidation inhibitor comprises a benzotriazole.
 19. The slurry as recitedin claim 14, wherein said additive is comprised of a sulfated fattyacid.
 20. The slurry as recited in claim 19, wherein said sulfated fattyacid has a molecular weight less than approximately
 300. 21. The slurryas recited in claim 19, wherein said sulfated fatty acid is selectedfrom the group consisting of sodium octyl sulfate, Duponol SP, andDuponol WN.
 22. The slurry as recited in claim 14, wherein said additivecomprises Duponol SP.
 23. The slurry as recited in claim 14, furthercomprising colloidal silica.
 24. The slurry as recited in claim 23,wherein said colloidal silica has particulates having a size less thanapproximately 0.4 microns.
 25. The slurry as recited in claim 14,wherein said slurry has a pH of approximately between 3.0 to 7.5. 26.The slurry as recited in claim 25, wherein said slurry has a pH of about4.5.
 27. A slurry for removing a tantalum-based barrier layer liner forcopper-based metallurgy comprising: about one liter of colloidal silicaslurry containing between 2 and 30 percent by weight solids in water; upto 10 ml/liter 30 percent aqueous hydrogen peroxide; between 1.5 and 6.0ml/liter sodium lauryl sulfate; up to 6.0 ml/liter surfactant; up to 4.0g/liter benzotriazole; and said slurry being adjusted to have a pH ofbetween 3.0 and 7.5.
 28. A slurry for removing a tantalum-basedbarrier/liner for copper-based metallurgy comprising: one liter ofcolloidal silica slurry containing 15 percent by weight silica; 3.0ml/liter 30 percent aqueous hydrogen peroxide; 3.0 ml/liter Duponol SP;1.2 g/liter benzotriazole balanced to a pH of 4.5.